Thermal spraying is widely used in various industrial fields as effective surface modification method. However, the coating failures such as cracking, debonding and spallation occur due to external loading or internal stress induced by the mismatch of material properties between coating and substrate, and these become a major problem. In this study, mechanical properties of thermal-sprayed ceramic coatings were investigated. Al₂O₃ and Y₂O₃-stabilized ZrO₂ (YSZ) coatings were deposited on plate substrates. Stainless steel plates and aluminum plates, of different thermal expansion coefficients, were used as substrates. The coatings were prepared at two different thicknesses. During deposition of each specimen, the history of substrate temperature was recorded. Four-point bending tests were carried out, while strains at the coating surface and the substrate surface were measured with strain gages. The apparent Young's modulus of the coating was determined using the composite beam theory. Subsequently, the rupture strain of the coating was measured by a three-point bending test. In addition, the residual stresses of the coating were determined by mechanical polishing. The relationships between the results of these tests and the each substrate temperatures during deposition were discussed.

View full abstract

Elastic wave transmission characteristics in unidirectional fiber-reinforced composites are studied based on the two-dimensional finite element analysis. The composite is assumed to be a lay-up of a finite number of monolayers, each of which contains a single row of fibers spaced at equal distance. Influences of the stacking number and misalignment of monolayers as well as the presence of coating layer around the fibers on the wave transmission spectra are demonstrated for unidirectional SiC-fiber-reinforced Ti-alloy composites. It is shown that the transmission coefficients fall to low values in certain bands of frequency, i.e., stop bands in terminology analogous to perfectly periodic structures. This feature is found to appear more clearly for the transverse wave incidence, irrespective of the misalignment of monolayers. The stiffness reduction of the coating layer is shown to shift the stop bands to lower frequencies, which can be a useful feature for the monitoring of the fiber/matrix interfacial damage.

View full abstract

In this paper, coating stresses in thermal barrier coating (TBC) changing with cyclic heating - cooling are measured with a curvature measurement device developed in this study. The coating system chosen in this study is a dual-layered structure, and it consists of a partially stabilized zirconia (PSZ) as the ceramic coating layer and CoNiCrAlY as the metal-bond coating layer. The specimen used here is a strip-plate shape with thin thickness (600μm) extracted chemically from carbon steel coated by a thermal spraying process. A cyclic heating-cooling test and a cyclic heating-cooling test with a dwelling time at the maximum temperature are conducted for the strip-plate specimen. Deflection and coating stresses are measured continuously under these cyclic tests, and thermal deformation mechanisms generating the deflection and coating stresses are discussed based on primitive knowledge using an elementary beam theory.

View full abstract

Effect of porosities on the fatigue properties of high-pressure die cast aluminium alloy is addressed. Three-dimensional shape and location of porosities in the specimen were inspected by the X-ray CT to determine which pore affects the fatigue crack. The tension-compression fatigue test was carried out and the CT inspection was also conducted for all the specimens before and after the fatigue test. By comparing the CT inspection with the SEM observation of the fracture surface, we confirmed that the crack initiation was restricted to shrinkage pores near the specimen surface, and the inner large pores were not the source of fatigue crack. But we found that a cluster of shrinkage cavities can initiate a crack and affect the fatigue fracture although it exists far inside. Multiple cracks were also found on the bumpy fracture surface, and they seemed to reduce the fatigue life by coalescing into a single large crack. Moreover, we demonstrated the successive CT inspection during the fatigue test, and showed its availability to detect the fatigue crack propagation behavior.

View full abstract

In this study, β-tricalcium phosphate (β-TCP), particles reinforced bioresorbable plastics poly-L lactide (PLLA) composites containing three different β-TCP contents were prepared by injection molding. In order to clarify effects of the PLLA crystallinity, the specimens were heat treated. The results of differential scanning calorimetry indicated that the PLLA crystallinity increased with increasing heat treatment temperature. Tensile tests were conducted on the specimens. The results show that the tensile moduli increased with increasing β-TCP contents and crystallinity. On the other hand, the tensile strength decreased with increasing β-TCP contents. In order to predict the stress-strain behavior, analyses based on micromechanics and damage mechanics were conducted. Analytical predictions were in good agreement with the experimental results and the effectiveness of the present analysis was confirmed.

View full abstract

Optical fiber sensors have many advantages for structural health monitoring and are often used to monitor the strain distribution of structures. However, using Fiber Bragg Grating (FBG) sensors with a general interrogation system, e.g. Wavelength Division Multiplexing (WDM), permits only the measurement of average strain within the gauge length and moreover, a large number of FBG sensors may be necessary to monitor overall stress concentrations. A distributed strain sensor with a higher spatial resolution and longer sensing length is therefore desired for the accurate and effective monitoring of stress concentration. To meet this need, we present a new strain measurement system with a long gauge FBG sensor based on the principle of Optical Frequency Domain Reflectometry (OFDR) which enables us to measure fully distributed strain at high spatial resolution. In this paper, we describe the principle and optical simulation model of our proposed measurement system and show the results of numerical calculations.

View full abstract